The invention relates to a device for controlling jet separation for rocket engine nozzles having a large section ratio and intended for operation under conditions of varying pressure.
To obtain high specific impulse at altitude, rocket engines are fitted with nozzles having a large section ratio. For a given engine, as it rises through the atmosphere, ambient pressure drops off, passing from atmospheric pressure at sea level to a low pressure that is a function of altitude. Nozzles are generally optimized as a function of the overall performance of a launcher, which leads to using nozzles that are overexpanded so as to provide maximum thrust at a xe2x80x9cmatchedxe2x80x9d altitude. Consequently, at atmospheric pressure as it exists at sea level, the expansion of gases in the nozzle is limited by the phenomenon of the jet separating from the wall of the diverging portion of the nozzle.
That phenomenon exists throughout an initial stage of flight that starts at liftoff and continues up to the matched altitude, which can be situated at about ten kilometers above the ground, for example, at which altitude thrust reaches its maximum because the static pressure Pe of the gases in the outlet section of the nozzle is then equal to ambient pressure Pa which is relatively low. Throughout this initial stage of flight, the static pressure of the gases in the outlet section of the nozzle is well below ambient pressure, thus leading to the phenomenon of the jet separating inside the nozzle, which phenomenon disappears at the matched altitude. During this initial stage, this phenomenon limits the expansion ratio of the gases, i.e. the ratio between the pressure P0 in the combustion chamber and the static pressure Pe of the gases in the outlet section of the nozzle.
A drawback of the jet separating inside the nozzle is to create mechanical forces in the separation zones that are harmful to the structure of the diverging portion.
To limit the effects due to the jet separating, various types of solution have already been proposed.
A first type of solution consists in fitting rocket engines with deployable nozzle systems. Documents U.S. Pat. No. 4,489,889, U.S. Pat. No. 4,779,799, and U.S. Pat. No. 4,676,436 describe relatively complicated nozzle systems which make it possible during flight to match the outlet section and the length of the nozzle as a function of pressure conditions. The systems described in those documents were developed for use with missiles or upper stage engines which present operating characteristics and dimensions that are different from those of main rocket engines. Those fragile and bulky systems are not designed to be jettisoned at the matched altitude and they are therefore difficult to apply to the main thrust nozzles of a rocket.
A second type of solution consists in proposing nozzles that comprise a system of ejectable elements, such as that described in document FR-A-2 503 794 which shows a nozzle having a plurality of diverging portions of increasing size nested one within another and jettisoned in succession as a function of altitude. Nevertheless, such a system is complex and runs the risk of damaging the diverging portions as successive parts are jettisoned. Furthermore, sealing between the ejectable diverging portions is a major problem since the gases tend to filter between said elements and to damage them on their outside faces which are not designed to be subjected to hot gases.
Another document, FR-A-2 705 739 proposes an ejectable device for reducing the outlet section of a nozzle in order to confine the jet inside the nozzle and thus limit axial fluctuations in the position of the line of separation. A difficulty encountered by that device is the thermal behavior of the ejectable element, which is not provided with cooling and which is subjected to temperatures close to those of the gases in the combustion chamber, i.e. about 3000 kelvins (K). It is therefore necessary to provide for injection of a low-temperature fluid into the separation zone inside the diverging portion in order to reduce the amount of heat received by the wall. That leads to problems of fluid consumption and of reliability in operation.
Another type of solution concerns nozzles fitted with air admission systems which, as disclosed in document U.S. Pat. No. 5,683,033, enable separation to be stabilized and enable the drag of the diverging portion to be reduced. That system is constituted by multiple moving elements (e.g. valves) difficult to make reliable in operation in the sound and vibration environment of a rocket engine. Furthermore, like the system described in document U.S. Pat. No. 5,450,720 which shows a nozzle in which separation is controlled by admitting ambient air through large longitudinal slots formed in the wall of the nozzle, that system of U.S. Pat. No. 5,683,033 for admitting ambient air suffers from the risk of additional combustion of the gases coming from the nozzle, with the consequence of heating taking place at the wall of the diverging portion, thereby likewise requiring special thermal protection.
The problem of jet separation in the nozzle can also be countered by devices for injecting gas at various heights within the diverging portion serving to fix the line of separation and make it regular. That kind of device is described in particular in documents U.S. Pat. No. 3,925,982, and FR-A-2,628,488. Those fluid injection devices nevertheless require complex valve and control equipment for controlling flow rate, which leads to a considerable increase in the weight of the diverging portion of the nozzle. Furthermore, since those devices consume fluid, they are of use only when a gas is available at low cost and of little use otherwise, i.e. only with bypass flow engines.
As disclosed in document FR-A-2,639,404, that solution consists in controlling separation in a nozzle having a large section ratio by the presence of a fluid barrier implemented at the end of the diverging portion. That device is effective, but it requires gas to be consumed whose expansion could have been used more usefully either in the main nozzle of a full flow engine, or else in nozzles having a larger section ratio for a bypass flow engine. In addition, that equipment is expensive, is necessarily heavy because of the presence of valves, and installing it at the bottom end of a nozzle gives rise to a large amount of nozzle inertia that is detrimental to the strength of the thrust chamber.
The invention seeks to remedy the above drawbacks and to provide a device enabling jet separation in a nozzle to be eliminated or reduced. The invention seeks to reduce and control the phenomenon of jet separation by means of a device which is passive and does not consume fluid, whether for cooling purposes or for jet control purposes, and which presents structure that is static, thereby guaranteeing that the device is simple and reliable.
These objects are achieved by a device for controlling jet separation inside a rocket engine nozzle, the device comprising a jettisonable annular structure for placing around the outside wall of the nozzle level with the gas outlet section thereof, said structure defining a radial extension around the nozzle so that in the presence of an outlet jet from the nozzle it creates a low pressure zone in the vicinity of the bottom face of the device, thereby reducing jet separation inside the nozzle.
The invention thus provides a device for controlling jet separation which is simple and effective and which can be applied to any rocket engine, whatever the cycle it uses, and which by virtue of its design can be installed while requiring only minor modifications to the structure of the diverging portion. The device presents the advantage of not weakening diverging portions, and on the contrary it serves to provide mechanical reinforcement for the bottom end of the device of the diverging portion, thereby limiting the harmful effects of the large amounts of deformation that appear, in particular while engines are starting or stopping. The device does not degrade engine performance. In addition, since it is external to the diverging portion, it is not subjected to the thermal stresses to which all other devices presenting surfaces in contact with the jet are exposed, and as a result it does not require any gas to be tapped for the purpose of its own cooling.
According to an aspect of the present invention, the annular structure is substantially frustoconical in shape sloping outwards and downwards.
The effect of creating a low pressure zone on the bottom face of the device is thereby increased.
More particularly, the annular structures presents a profile with curvature that forms an angle with a plane perpendicular to the axis of the nozzle which lies in the range 10xc2x0 to 20xc2x0.
In an embodiment of the invention, the bottom face of the annular structure has steps for braking the flow of air along said face.
In which case, the extent to which the flow of air is slowed down over the bottom face of the device is further increased.
The annular structure can present an inside diameter that is slightly greater than the outside diameter of the outlet section of the nozzle, thereby leaving clearance enabling the nozzle to move relative to the annular structure under the effect of thermal expansion.
The annular structure is constituted by a set of angular sectors assembled together via contact surfaces which ensure that they move as a whole, the device further comprising means for holding the assembled-together sectors around the nozzle.
Such a structure for the device enables the annular structure to be jettisoned without any risk of damaging the diverging portion of the nozzle.
More particularly, the means for holding the sectors around the nozzle comprise a first cable surrounding the sector assembly in the vicinity of its inner periphery, and a second cable surrounding the sector assembly in the vicinity of its outer periphery, each of said first and second cables being connected to respective first and second tensioning members, thereby forming two sector-clamping loops, said tensioning members also comprising means for rupturing said cables.
The two cables exert radial forces on each sector, which forces are taken up by lateral forces serving to stiffen and reinforce the annular structure around the nozzle while still allowing it to be ejected when the time has come.
The contact surfaces between the sectors include portions which project upwards from the top faces of said sectors.
The risks of a sector buckling under contact pressure is thus avoided.
In the event of slip between the sectors being detected, the contact surfaces of the sectors can comprise alternating grooves and tongues for receiving or engaging respective tongues or grooves on the contact surfaces of adjacent sectors.
The grooves can advantageously contain respective elastomer gaskets for avoiding leaks between the sectors.
In a particular aspect of the invention, at least two sectors of the annular structure are smaller in size than the other sectors, the smaller-sized sectors being disposed uniformly in the annular structure.
Jet separation in the nozzle is then controlled by avoiding the appearance of disorganized separation zones which are dangerous for the structure of the diverging portion.
Jet separation in the nozzle can also be imposed with at least two sectors of the annular structure being perforated over their respective surfaces, the perforated sectors being disposed uniformly within the annular structure.
Controlled separation of the jet then enables the nozzle to be appropriately dimensioned.
The invention also provides a rocket engine nozzle including a jet separation control device as defined above.
More specifically, the nozzle comprises a substantially plane outwardly-directed rim of diameter greater than the diameter of the inside periphery of the device so as to form a support for said device.
This is the minor modification that needs to be made to a rocket engine nozzle so as to enable it to be used with the device of the invention.
More precisely, the rim extends radially from the outlet section of the nozzle over a distance of the order of 5 centimeters (cm) to 8 cm.
In accordance with an aspect of the invention, the outlet section of the nozzle with the rim is covered on its outside surface in a material that is deformable.
In this way, any leaks between the device and the outside wall of the nozzle can be limited.